J Oral Maxillofac 49:2&I-267,

Surg

1990

The Effect of Repetitive Screw Hole Use on the Retentive Strength of Pretapped and Self-Tapped Screws WILLIAM

L. FOLEY, DMD,* DAVID E. FROST, DDS, MS,-/- AND MYRON R. TUCKER, DDSS

Multiple systems for internal screw fixation are presently in use in oral and maxillofacial surgery. These systems differ in screw diameter, material, and design. This study evaluated the uniaxial pullout strength of five screw systems. Two-millimeter, 2.7-mm, and 3.5mm screws were initially evaluated for pullout strength. Pretapped and self-tapped screws were then inserted into the same hole multiple times before pullout testing. Results of pullout testing revealed no significant difference in pullout strength of pretapped and self-tapped screws of equal diameter (P > .05). Increased screw diameter produced increased pullout strength. No significant difference in pullout strength was noted in pretapped or self-tapped screws inserted into the same hole one, two, or three times before pullout testing (P > .05).

screw design differs, they can be broadly classified as self-tapped or pretapped. Materials most commonly used are stainless steel, vitallium, and titanium . Much of the pioneer work in internal screw fixation has been accomplished through the activities of the Swiss Arbeitsgemeinschaft fur Osteosynthesefragen (AO), also known as the Association for the Study of Internal Fixation (ASIF). The A0 philosophy has long been that pretapped screws are preferable to self-tapped screws.‘** In pretapped screw systems a twist drill is used to create a hole in the bone, and a tap is then used to cut threads in the bony wall. Theoretical advantages of this system include the ability to remove a screw and return it to the same hole without loss of retentive strength and the ability to reduce the torsional force developed by the screw as it is inserted.‘*3.4 Removing a screw and replacing it in the same hole without loss of retentive strength makes compression plate adaptation and reconstruction plate placement easier. Additionally, Schilli’ asserts that tapping the screw hole offers greater and more even bone contact between the screw and bone surface, spreading the force exerted by the screw over a larger area of bone. Self-tapping systems are also widely used for

With the advent of small compression plates, improved instrumentation, and antibiotic therapy, the use of internal screw fixation (ISF) in fracture management and reconstructive and orthognathic surgery has gained renewed interest in recent years. Several different systems for ISF have been developed. They differ in screw diameter, design, and material. Screw diameters most commonly used today in maxillofacial surgery include 2.0 mm, 2.7 mm, and 3.5 mm. Although each manufacturer’s Received from the University of North Carolina at Chapel Hill. * Chief Resident, Oral and Maxillofacial Surgery; Major, USAF, DC. f Clinical Assistant Professor for Research and Part-Time Faculty; in private practice. $ Assistant Professor, Director of Graduate Training. This report was the winner of the 1989 Merck Sharp & Dohme Resident Research Award. The opinions and assertations are those of the authors and not to be thought of as reflecting the views of the United States Air Force or the Department of Defense. This research was supported by local university funding and NIDR Grant No. DE05215. Address correspondence and reprint requests to Dr Frost: 401 Providence Rd, Chapel Hill, NC 27514. 0 1990

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Oral

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Maxillofacial

Sur-

geons

0278-2391/90/4803-0007$3.00/0

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FOLEY, FROST, AND TUCKER

ISF. In these systems a twist drill is used to prepare a hole in the bone. The screw is then placed directly into the hole, eliminating the tapping step and requiring less armentarium and operating time. A theoretical disadvantage of self-tapped screws is the loss of retentive strength of the screw if it is removed and then replaced in the same hole. Selftapped screws also require increased torque for placement.3*4 This study compares the retentive (uniaxial pullout) strength of five commonly used fixation systems. These systems vary in screw size, thread design, material, and placement technique. This study also evaluates the hypothesis that removing and replacing pretapped screws in the same hole does not decrease retentive strength, and that removing and replacing self-tapped screws in the same hole decreases their retentive strength. Materials and Methods Porcine rib bone was used for this study. The bone was stored in physiologic saline and refrigerated until all tests were completed. Testing was completed within 5 days of the animals’ death. Ko’ reported no significant change in the tensile strength of human bone stored in physiologic saline at room temperature for several months. Hazama6 found the same results for torsion strength of human compact bone. The bones were stripped of soft tissue and cut into 14-cm segments. Screws were placed at &mm intervals along the bone. Each test trial consisted of a bone with three screws from each group placed along its length. The first screw of each group was placed in the standard fashion (single insertion). The second screw was placed in the standard fashion, removed, and then replaced in the same hole (double insertion). The third screw in each group was placed, removed, and replaced in the same hole twice before pullout testing (triple insertion). The position of the groups was varied in each subsequent trial so that each screw type was placed in a different position along the bone. A total of 10 trials were performed so that 10 screws of each technique and 30 screws of each type were tested. The groups compared were as follows: group WST consisted of 2.0-mm self-tapped Wurzburg screws. Screw holes were prepared with a 1S-mm twist drill. Group RPT consisted of 2.0-mm pretapped Richards screws. Screw holes were prepared with a 1S-mm twist drill and tapped with a 2.0-mm tap. Group WPT consisted of 2.7-mm pretapped Wurzburg screws. Screw holes were prepared with a 2.0-mm twist drill and tapped with a 2.7-mm tap. Group LST con-

sisted of 2.7-mm self-tapped Luhr screws. Screw holes were prepared with a 2.0-mm twist drill. Group OPT consisted of 3.5mm pretapped Richards screws. Screw holes were prepared with a 2.0mm twist drill and tapped with a 3.5-mm tap. Groups tested are summarized in Table 1. Screw holes were prepared with a slowly rotating twist drill in an electric hand drill. The hand drill was stabilized in a drill press. All screws were placed by the same investigator (W.L.F.) 90 degrees to the bone surface through one cortex of the rib to a depth of 7 mm. The bone specimens were secured in a custommade jig, and screws were subjected to tensional forces at a predetermined rate of increase (cross head speed 5 mm/min). The screw heads were grasped by specially designed jigs, and an Instron machine was used to subject the screws to tension loading to the point of failure. The Instron machine (Instron Cot-p, Canton, MA) is a precision instrument that uses a screw-driven movement to generate a variety of loads measured on a precision strain gauge. The yield point of each screw was recorded in kilograms of tension applied. The cortical thickness of the bone was recorded at each screw location with a depth gauge and was uniformly 2 to 3 mm thick. Results Results of the uniaxial pullout tests showed that in all cases failure occurred at the metal-bone interface, with bone splintering around the screw threads. Analysis of variance and Tukey pairwise comparisons with experimentive level of significance set at .05 showed no difference (P > .05> in pullout strength of pretapped or self-tapped screws of equal diameter (Fig 1). Increased screw diameter produced increased pullout strength (Table 2). Screws with diameters of 2.7 mm and 3.5 mm demonstrated significantly greater retentive strength than 2.0-mm screws (P < .05). The distinction between the 3.5-mm screw group and the 2.7-mm screw groups was less well defined and could not be demonstrated statistically. Table 1. GTOUP

WST RPT LST WPT OPT

Screw GroupsTested Screw Type

Diameter (mm)

Placement Technique

Wurzburg Richards Luhr Wurzburg Richards

2.0 2.0 2.7 2.7 3.5

Self-tapped Pretapped Self-tapped Pretapped Pretapped

266 PULLOUT 45.0

EFFECT OF REPETITlVE

(kg)

PULLOUT

SCREW HOLE USE

(kg)

46 42 36

30.0

30

22.5

24

15.0

18

12 7.5

6 i-l _

0.0 2.0 El

WST 2.0mm

2.0

2.7

GROUP OIIII RPT 2.0mm EI LST 2.7mm

2.7 0

WPT 2.7mm

Results of multiple screw insertions before pullout testing were analyzed using repeated-measures ANOVA. No significant difference (Z’ = .7) in pullout strength was noted in pretapped or self-tapped screws after insertion into the same hole once, twice, or three times before pullout testing (Fig 2).

Uniaxial pullout tests provide an accurate method of comparing retentive strength in a uniaxial direction. Using this method, we found no significant difference in pullout strength of pretapped and self-tapped screws of equal external diameter. These results are consistent with the in vitro findings of Koranyi et al,’ and the in vivo findings of Schatzker et aI,’ who used a pushout test to determine retentive strength. These findings are also consistent with the in vitro findings on Vangsness et al9 and the in vitro findings of Foley et al. ” Increased screw diameter from 2 mm to 2.7 mm produced increased pullout strength. This finding is again consistent with the findings of Koranyi et al’ and Phillips and Rahn.4 We were unable to demonstrate a statistically significant difference, however, in pullout strength between 2.7-mm screws and 3.5mm screws. This could be due to the thin cortical thickness (2-3 mm) of the bone used in this study. Phillips and Rahn,4 using varying thicknesses of calf femur, demonstrated that improved compressive forces achieved using 2.7-mm screws, as compared with 2.0-mm screws, did not become apparent until Unirxial Mean Pullout Strength

Group

Diameter (mm)

WST RPT LST WPT OPT

2.0 2.0 2.7 2.7 3.5

Values are given as mean 2 SD,

RPT

LST



WPT

OPT

GROUP

FIGURE 1. Mean uniaxial pullout strengths of pretapped versus self-tapped screws having the same diameters.

Table 2.

WST

Pullout Strength 28.58 26.17 37.97 41.38 44.28

k k + + *

11.27 12.21 13.33 10.51 16.46

EZI Single

Insertion

E0 Double

Insertion

DTriple

Insertion

FIGURE 2. Mean uniaxial pullout strengths of pretapped and self-tapped screws after multiple insertions. WST = 2.0 mm; RFT = 2.0 mm; LST = 2.7 mm; WPT = 2.7 mm; OPT = 3.5 mm.

3- and 4-mm bone thicknesses were tested. In vitro research has shown that increased cortical thickness produces an increase in pullout strength.7*9Y1’ The cortical thickness used in this study did not vary enough to verify previously reported findings. The findings in this study do not support the contention that removing and replacing a self-tapped screw in the same hole decreases its retentive strength. Removing either a self-tapped screw or a pretapped screw and replacing it in the same hole did not significantly change its retentive strength. This finding is consistent with that of Schatzker et al,* who stated that self-tapped screws inserted in the same hole up to 12 times at 80% of their torqueout value showed no significant difference in pushout resistance. However, Schatzker provided no statistical data to support his findings. Schilli’ has stated that pretapped screw holes offer greater and more even bone contact between the screw and bone surface, spreading the force exerted by the screw over a larger area of bone. Uhthoff” and Phillips and Rahn4 have shown that taps have been found to be larger than the external diameter of the screw, thereby decreasing thread contact with the bone. Uhthoff’* demonstrated histologically that only part of the horizontal thread surface (facing the head of the screw) was in intimate contact with the bone. Both the oblique surface, facing toward the tip of the screw, and the vertical surface between the neighboring threads were separated from the bone by interposed soft tissue up to 150 pm thick. Schatzker’ reported that no histologic differentiation could be made with regard to both cell death or tissue reaction around implants placed with self-tapped or pretapped insertion methods.* The only significant difference between pretapped and self-tapped screw is the torsional force required to place self-tapped screws. This study has demonstrated that no difference in pullout strength exists between pretapped and self-tapped screws after one, two, or three placements in the same hole.

FOLEY. FROST, AND TUCKER

Self-tapped screws should perform as well as pretapped screws in those aspects of oral and maxillofacial surgery where increased torsional forces are not a factor. Acknowledgment The authors wish to thank Ceib Phillips, PhD, for statistical assistance with this report. We also wish to acknowledge Walter Lorenz, Inc, and Howe-Medica Corp. for donation of equipment used in the research.

References 1. Schilli W: C’ompression osteosynthesis.

J Oral Surg 35:802, 1917 2. Brons R. Boering G: Fractures of the mandibular body treated by stable internal fixation: A preliminary report. J Oral Surg 28:407, 1970 3. Hughes AN, Jordan BA: The mechanical properties of surgical bone screws and some aspects of insertion practice. Injury 4~25, 1972 4. Phillips JH, Rahn BA: Comparison of compression and torque measurements of self-tapping and pre-tapped screws. Plast Reconstr Surg 83447, 1989

267 5. Ko R: The tension test upon the compact substance of the long bones of human extremities. J Kyoto Pref Med Univ 53:503, 1953 6. Hazama H: Study on the torsional strength of the compact substance of human beings. J Kyoto Pref Med Univ 60: 167, 1956 7. Koranyi E, Bowman CE, Knecht CD, et al: Holding power of orthopedic screws in bone. Clin Orthop Rel Res 72:283, 1970 8. Schatzker J, Sanderson R, Mumaghan JP: The holding power of orthopedic screws in vivo. Clin Orthop Rel Res 108:115, 1975 9. Vangsness CT, Carter DR, Frankel VH: In vitro evaluation of the loosening characteristics of self-tapped and nonself-tapped cortical bone screws. Chn Orthop Rel Res 157279, 1981 10. Foley WL, Frost DE, Paulin WB, et al: Uniaxial pull-out evaluation of internal screw fixation. J Oral Maxillofac Surg 41~277, 1989 11. Perren SM: Physical and biological aspects of fracture healing with special reference to internal fixation. Clin Orthop 138:175, 1979 12. Uhthoff HK: Mechanical factors influencing the holding power of screws in compact bone. J Bone Joint Surg 558:663, 1973

The effect of repetitive screw hole use on the retentive strength of pretapped and self-tapped screws.

Multiple systems for internal screw fixation are presently in use in oral and maxillofacial surgery. These systems differ in screw diameter, material,...
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